Australia Telescope National Facility

New Structure In The Shapley Supercluster
M.J.Drinkwater, D.Proust, Q.A.Parker, H.Quintana, E.Slezak, PASA, 16 (2), in press.

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Introduction

The Shapley supercluster (SSC) has been investigated by numerous authors since its discovery in 1930 (Quintana et al. 1995; hereafter Paper I). It lies in the general direction of the dipole anisotropy of the Cosmic Microwave Background (CMB), and is located at 130

$\rm\thinspace h^{-1}_{75}Mpc$ beyond the Hydra-Centaurus supercluster ( $\simeq 50$

$\rm\thinspace h^{-1}_{75}Mpc$ away from us). It consists of many clusters and groups of galaxies in the redshift range 0.04<z<0.055. The central cluster A3558 has also been measured with a ROSAT PSPC observation by Bardelli et al. (1996) who derive a total mass of

$M_{tot}=3.1\times10^{14}\hbox{${\rm\thinspace M}_{\odot}$}$ within an Abell radius of 2

$\rm\thinspace h^{-1}_{75}Mpc$ . Several other x-ray clusters form part of the Shapley supercluster (Pierre et al. 1994).

The Shapley supercluster is recognised as one of the most massive concentrations of galaxies in the local universe (Scaramella et al. 1989; Raychaudhury 1989), so it is of particular interest to consider its effect on the dynamics of the Local Group. In Paper I it was estimated that for

$\Omega_{o}= 0.3$ and

$H_{o}= 75\hbox{$\rm\thinspace km s^{-1} Mpc^{-1}$}$ the gravitational pull of the supercluster may account for up to 25% of the peculiar velocity of the Local Group required to explain the dipole anisotropy of the CMB radiation, in which case the mass of the supercluster would be dominated by inter-cluster dark matter.

Previous studies of the Shapley supercluster (Paper I, Quintana et al. 1997; hereafter Paper II) have concentrated on the various rich Abell galaxy clusters in the region, but this might give a very biased view of the supercluster. As was noted in Paper I, ``the galaxy distribution inside the supercluster must be confirmed by the detection in redshift space of bridges or clouds of galaxies connecting the different clusters''.

We are continuing this project, using data from wide-field multi-fibre spectrographs to measure many more galaxy redshifts and get a more complete picture of the composition of the supercluster. Our main aims are first to define the real topology of the SSC: in Paper I it was shown that the SSC is significantly flattened, but the real extent of the concentration is not well defined. Secondly we will analyse the individual X-ray clusters that are true members of the Shapley Supercluster in order to estimate the cluster masses, and investigate suspected sub-structure. Additional observations are planned before we present a full analysis of the dynamics (Proust et al. 1998 in preparation).

In Paper II we presented data from the MEFOS spectrograph on the European Southern Observatory 3.60m telescope. This has 30 fibres in a 1 deg diameter field, so the observations were again mainly concentrated on the known clusters, determining for several of them if they were members of the supercluster or not.

In this paper we present new data obtained with the FLAIR-II (Parker & Watson 1995; Parker 1997) multi-fibre spectrograph on the UK Schmidt Telescope at the Anglo-Australian Observatory. This has 90 fibres in a $5.5\times5.5$ deg² field and has allowed us to measure a much more uniform distribution of galaxies in the direction of the SSC, avoiding the previous bias in favour of the rich clusters. Our data reveal the existence of a sheet of galaxies connecting the main parts of the supercluster. We describe the sample and observations in Section 2. We present the results along with previous measurements in Section 3 and discuss the significance of the measurements in Section 4.

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